Work with one piece of dough at a time. Using your fingers, gently press it into a rectangle about 5 x 2.5 inches.
Brush some melted butter onto the top of the rectangle, leaving a ¼-inch border around the edges unbuttered (see notes). Set aside the remaining butter—this will be used later for brushing the tops of the baked rolls.
Fold the rectangle almost in half, so one end of the unbuttered edge stays exposed. Holding the folded dough in one hand, use your other hand to gently stretch the unbuttered edge outward, then fold it into the center. Press it down firmly with your finger to help seal it.
Rotate the dough slightly (about 45 degrees), stretch a new edge, and fold it into the center, pressing firmly again. Repeat this process (rotate, stretch, fold, and press) about three times. This helps shape the dough into a smooth, taut ball. If needed, pinch the bottom to help it seal.
Lastly in forming the bun if the shape needs a little work, place the dough ball on the work surface (seam side down) and using your cupped hand roll the dough ball gently until it has a nice shape. Place in the greased 9x13 pan seam side down (see notes).
Keep the buttered 9x13 pan covered with plastic wrap while getting all 15 rolls into the pan. Once filled, Let the rolls rise one more time at room temperature for about 45 minutes so they can puff up a bit and are touching each other? Thirty minutes into this second rise, preheat the oven to 350F.
Prepare the egg wash by whisking together one egg and a teaspoon of water.
Once the rolls have finished rising the second time, remove the plastic wrap and brush the tops of the rolls with the egg wash. Place the pan on the middle rack of the oven and bake for 20-30 minutes. The tops should be golden brown. Depending on your oven you may want to rotate the pan halfway through baking to get an even browning. You can also insert a thermometer into one of the buns to check to see if they are done, it should read 190F.
Remove from the oven and brush on more melted butter and sprinkle with flaky salt.
Allow the rolls to cool in the pan for about 20 minutes before serving warm.